In-situ synchrotron X-ray radiography study of primary Fe-rich phases growth in Al-Fe(Cu) alloys

Plate-like iron-rich intermetallic phases (Fe-rich phases) greatly decrease the elongation and fatigue strength of recycled Al alloys. The alloy composition and the manufacturing process parameters influence the type, and shape of Fe-rich phases. In this study, the influence of the cooling rate (0.1, 0.5, and 1.0 °C/s) on the nucleation and growth of primary Fe-rich phases during solidification of Al-Fe(Cu) alloy has been quantified by synchrotron X-ray radiography. The Kikuchi patterns and deep-etched methods showed that rod-shape and plate-like primary Al3Fe phases are present in the studied alloy. The in-situ experimental results indicate that the total number and number density of primary Fe-rich phases in AlFe alloys is very similar at cooling rates of 0.1 °C/s and 1.0 °C/s but increases a 300 % at cooling rate of 0.5 °C/s. The nucleation rate of Al3Fe in AlFe alloys at three cooling rates (0.1, 0.5, and 1.0 °C/s) are 15.5, 44.0, 25.0 No./s, respectively. These decrements are due to alloy compositions are very closed to the eutectic composition (about 1.8% Fe), the less time for them to form primary Fe-rich phases at high cooling rate of 1.0 °C/s. The variation trend of total number, number density, and nucleation rate of primary Al3Fe phases in Al-Cu-Fe alloys is similar to those of in AlFe alloys. The maximum length of typical Al3Fe phases in AlFe alloys decrease from 845 μm to 222 μm as the cooling rate increased. This is because that higher cooling rate resulting in less diffusion time for limiting Fe atoms diffusion, also inhibiting them grow into large plates. The combined Cu addition and higher cooling rate resulting in the constitutional undercooling, and finally promote the formation of refined primary Al3Fe particles.